A direct-injection (DI) engine may emit increased particulate matter (PM) under certain operating conditions. During a cold start, for example, fuel injected into the combustion chambers of the engine may be inadequately vaporized, requiring excess fuel injection to achieve stable combustion. The excess fuel may result in an over-rich air-to-fuel ratio, which causes increased PM emission. This issue may be exacerbated when the engine is significantly loaded during the cold start, as higher engine load results in increased manifold air pressure (MAP), which may reduce the degree of atomization and vaporization of the injected fuel.
To remedy this issue, U.S. Pat. No. 6,769,400 provides for a vehicle an electric propulsion system in parallel with a combustion propulsion system. The electric propulsion system is engaged to reduce the MAP of the combustion propulsion system to a predetermined pressure before initiating the combustion propulsion system. While the strategy proposed in the reference may reduce emissions—possibly including PM emissions—in hybrid vehicles, such active suppression of MAP may delay acceleration in turbocharged engine systems.
The inventors herein have recognized these issues and have proposed a series of solutions to address them. Accordingly, one embodiment provides a method for operating an engine of a vehicle, comprising: during a warm-up period following a cold start of the vehicle and in response to insufficient atomization of directly injected fuel causing increased particulate emissions, delaying a transmission up-shift to a higher driver-demand level than would otherwise cause the up-shift for a given condition. In this way, for a given acceleration profile, the delayed up-shift provides a greater degree of operation (e.g., number of combustion events) at higher speeds/lower loads, and thus a greater degree of operation with lower manifold pressure as compared to the shift timing that would otherwise occur. In this way, reduce emissions can be achieved, at least over a portion of the acceleration profile where the shift is delayed.
Another embodiment provide a method for operating an engine of a vehicle in response to a driver demand. During a warm-up period following a cold start of the vehicle, a cylinder of the engine is charged to a reduced air-charge pressure, and fuel is injected into the cylinder. After the warm-up period, the cylinder is charged to a non-reduced air-charge pressure, greater for an equivalent driver demand than the reduced air-charge pressure, and fuel is injected into the cylinder. In another embodiment, a speed threshold at which an automatic transmission of the vehicle shifts to a higher gear is increased during the warm-up period. After the warm-up period, the speed threshold is lowered. The warm-up period may last until fuel, if injected into a cylinder charged to the non-reduced air-charge pressure, would be vaporized at target efficiency. In this manner, the air-charge pressure in the cylinder is reduced during the warm-up period, resulting in better atomization of the fuel and lower PM emissions.
The summary above is provided to introduce a selected part of this disclosure in simplified form, not to identify key or essential features. The claimed subject matter, defined by the claims, is limited neither to the content of this summary nor to implementations that address problems or disadvantages noted herein.